Method and installation for producing breathable air

ABSTRACT

The invention relates to a process for producing respirable air comprising the following stages: treatment of compressed air comprising an air-drying operation rehumidification of the treated dry air.  
     According to the invention, the rehumidification stage of the treated dry air comprises an operation of controlled distribution of the treated dry air on the one hand in a rehumidification line ( 24 ), and on the other hand in a dry line ( 22 ).  
     The invention also relates to an installation ( 1 ) for utilising such a process. Application to the area of nuclear installation dismantling.

TECHNICAL FIELD

The present invention relates to the field of processes for theproduction of respirable air comprising a treatment stage for compressedair, the latter in addition comprising an operation for drying the air.

More particularly, the invention concerns processes for production ofrespirable air to be used by operators carrying out works on sensitivesites, such as for example works for dismantling nuclear plants, oragain works for removing asbestos. Also by way of example, the airproduced by such processes can also be for medical use.

The invention also relates to production installations for respirableair likely to utilise such processes.

PRIOR ART

In conventional processes of production of respirable air, a stage fortreating compressed air supplied by one or more compressors is firstcarried out, such that the maximum of impurities is extracted from theair consumed by users.

To do this, carbon monoxide is essentially trapped by means of acatalyst, this gas being obtainable in highly significant quantities incompressed air coming from compressors. The harmful presence of this gasand other such as carbon dioxide can especially result from variousmalfunctions of air compressors being used, or again from the proximitybetween the aspiration of the compressors and these different gasescontained in the atmosphere.

It is noted that in the case of dismantling work on nuclearinstallations, the air breathed by the operators must respect certaincharacteristics, itemised in the standard NE EN 12021. In this respect,this standard indicates that the maximum admissible value of carbondioxide in respirable air is 500 ppm, and that the maximum admissiblevalue of carbon monoxide in respirable air is 15 ppm.

During the treatment stage of compressed air, a drying operation of theair is undertaken by adsorption, with a dew point of between −40° C. and−70° C.

During this aspiration, the quasi-totality of the carbon dioxide istrapped, whereas all trace of humidity in the air is reduced. Thisallows the catalyst used for trapping the carbon monoxide to functioncorrectly.

In this type of process, the respirable air produced responds to thespecifications of the abovementioned standard, but all the same poses amajor drawback.

In effect, because of the drying operation performed during thetreatment stage of the previously described process, the air produced isvery dry. Consequently, it is likely to cause desiccation of therespiratory organs in the operators consuming this air.

To reply to this problem, it has been proposed to add a rehumidificationstage of the treated air, so that the supplied air has a humidity raterelatively similar to that of the air aspirated by the compressors.

This type of process is especially mentioned in the document U.S. Pat.No. 4,054,428.

This process is utilised by an installation comprising two chamberscontaining agents enabling compressed air to be dehumidified. As thecompressed air passes into the first of these chambers, the air isdried, and then it passes through a space in which the carbon monoxideis transformed into carbon dioxide. The dehumidified air then circulatesin the second chamber of the installation, where it is rehumidified bymeans of agents contained in this second chamber, having absorbedhumidity during a previous cycle.

To utilise this process, the installation also comprises a four-wayvalve, allowing the direction of flow of compressed air to be invertedacross the installation, so that this compressed air circulatesalternatively from the first to the second chamber, and from the secondto the first chamber. Note that this recurring inversion of direction ofthe flow of compressed air across the installation is a necessarycondition for obtaining rehumidification of the air produced.Accordingly, this installation seems only slightly adapted to thecontinuous production of air, and in no case allows the production ofrespirable air at a constant rate of humidity, over a significantperiod.

In addition, this type of process comprises a certain number of majordisadvantages, especially including that of the complexity of theinstallation utilised, or again that of the incapacity of regulating therate of humidity of the respirable air produced. Another disadvantage isthe risk of desorption of carbon dioxide, recovered as the air passesthrough the column for rehumidifying.

EXPLANATION OF THE INVENTION

The first object of the invention is to propose a process for producingrespirable air, at least partially eliminating the disadvantages of theprocesses of the prior art mentioned hereinabove.

In addition, another object of the invention is an installation forproduction of respirable air, for executing a process such as thatresponding to the object mentioned hereinabove.

To achieve this, the primary object of the invention is a for productionof respirable air comprising the following stages:

-   -   treatment of compressed air comprising an air drying operation;    -   rehumidification of the treated dry air.

According to the present invention the re-humidification stage of thetreated dry air comprises an operation for controlled redistribution ofthe treated dry air on one hand in a rehumidification method, and on theother hand in a dry method.

Advantageously, the process according to the present invention producesrespirable air at a regulatable and constant rate of humidity,irrespective of the rate of air to be produced.

Preferably, the distribution of treated dry air is controlled by meansof a regulating valve mounted on the rehumidification line andcontrolled by pilot means sensitive to the signal output by a probemeasuring the rate of humidity, the probe being mounted on an outletpipe connected at one end to the rehumidification line, and at the otherend to the dry line.

In addition, a difference can be made in pressure between therehumidification line and the dry line, so as to favour the passage oftreated dry air originating from the dry line, in the outlet pipe. Thus,the probe measuring the rate of humidity will not be wet excessively,the consequence of which would be to render it inoperable.

Preferably, the stage for treatment of compressed air comprises thefollowing operations:

-   -   filtering condensates found in the compressed air;    -   drying the air to eliminate any trace of humidity in the air;    -   filtering dust dislodged during the drying operation;    -   transformation of carbon monoxide contained in the compressed        air into carbon dioxide;    -   filtering the air using an active carbon filter.

In a preferred manner, the stage for treatment of the compressed air isfollowed by a permanent stage of analysis of quantities of carbonmonoxide and carbon dioxide present in the treated air, then an alertstage when the values of these quantities exceed maximum values to beobserved.

Finally, after the rehumidification stage of the treated dry air, theair has a rate of humidity of between approximately 40 and 50%, and canbe provided to feed at least a ventilated suit of an operator carryingout dismantling works for nuclear plants.

Yet another object of the invention is an installation for production ofrespirable air comprising:

-   -   means for processing compressed air comprising means for drying        air;    -   means for rehumidification of dry treated air.

According to the present invention the means for rehumidification of thetreated dry air comprise a rehumidification line and a dry line, as wellas distribution means for controlled distribution of the treated dry airin each of the lines.

Other characteristics and advantages of the invention will emerge fromthe following detailed, non-limiting description.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be given with reference to the sole attachedFIGURE, illustrating a schematic view of an installation for productionof respirable air according to a preferred embodiment of the invention.

DETAILED EXPLANATION OF A PREFERRED EMBODIMENT

In reference to the sole FIGURE, the present invention concerns aninstallation 1 for production of respirable air by humans, for use on anindustrial site where operations generating ambient air pollution airare carried out, by fumes, dust, vapours, in particular on premises, ina room or a closed structure.

Preferably, the installation 1 for production of respirable air isapplied in the area of dismantling nuclear plants, and the operatorscarrying out the works are constrained from carrying ventilated suits,so as to avoid being in contact with contaminated zones.

It should be noted that the description to be given for an installation1 for production of respirable air to be connected to ventilated suits(not shown) of operators carrying out dismantling work on nuclearplants, but that of course the installation 1 and the process formingobjects of the invention apply equally to areas other than nuclear.

By way of examples, the invention could also find application on worksites for removing asbestos which generate particles and asbestos dustlikely to be carcinogenic, on work sites where painting operations arecarried out, or again at sites where welding or metal cutting arecarried out, with substantial emission of smoke.

The installation 1 is fed with compressed air by air compression means(not shown) allowing the air to be compressed at a pressure greater than1 bar, and preferably between 9 and 15 bars. In addition, the aircompression means are adapted to supply a rate of compressed air ofbetween 10 m³/h and 1000 m³/h per installation.

For example, the air compression means can take the form of compressorslubricated by screws or pistons, or again take the form of dry-screwcompressors.

The compressed air leaving the compression means is usually loaded witha multitude of impurities, such that it is necessary to extract beforedirecting this air to the different ventilated suits of the operatorsworking on the site.

The harmful substances giving the greatest concern are carbon monoxide(CO) and carbon dioxide (CO₂), which when present in excessivequantities within a ventilated suit, can engender catastrophicconsequences for an operator wearing this suit.

The origins of the presence of such substances in the compressed airexiting from the compression means are diverse and varied. By way ofexample, this can be a question of the defective character of aseparator filter of a lubricated compressor, the rupture of the coolingcircuit of a dry-screw compressor, or the simple presence of these gasesin the atmosphere in proximity to the aspiration of the compressionmeans.

Furthermore, a European standard NE EN 12021 indicates the maximumvalues on CO and CO₂, which can make up the air to be breathed.

As for CO₂, the maximum admissible value imposed by this standard is 500ppm (particles per million), this low value being adapted so that theair produced approaches the maximum of natural air, generally containingin the region of 400 ppm of CO₂.

In the same way, for CO, the maximum value imposed by this standard is15 ppm, this gas known to be extremely toxic.

To eliminate as much as possible the harmful substances contained in thecompressed air supplied to the installation 1, the latter firstcomprises processing means 2 for compressed air, especially allowingdrying of the air to be redistributed.

In addition, because of the possibility of desiccation of therespiratory passages of humans breathing the treated dry air exitingfrom the processing means 2, the installation 4 has rehumidificationmeans 4 of the treated dry air, connected to the processing means 2.

The different elements making up the processing means 2 will now bedescribed, in the order corresponding to that in which the compressedair encounters these elements, as it passes through the installation 1.

The processing means 2 first comprise a separator filter 6 at 0.01 ppm,whereof the essential role is to trap the condensates in the compressedair. The filter 6 is equipped with an automatic purge electrovalve 8,for evacuating the different filtered substances. The filter 6 isattached at one end to a pipe 9 communicating with the air compressionmeans (not shown), and at the other end to a pipe 10 communicating alsowith drying means of the dryer adsorption type lI.

The aim of the adsorption dryer 11, with a dew point of −73° C. underpressure, is to eliminate any trace of humidity in the compressed air.Note that the dryer 11 comprises a molecular screen (not shown) trappingthe quasi-totality of the CO₂ contained in the compressed air.

Connected directly to the adsorption dryer 11 by means of a pipe 12, theprocessing means 2 have a filter 13 of 1 micron particles, whereof theprincipal function is to stop the dust dislodges by the dryer 11.

In addition, the presence of a catalyst 14 of CO—CO₂ connected to thefilter 13 by means of a pipe 15 can be noticed, this catalyst beingcapable of retaining the CO by means of the hopcalite (mixture ofmetallic oxides), and catalysing the transformation of the carbonmonoxide into carbon dioxide. It should be specified that the adsorptiondryer 11 is placed upstream of the catalyst 14, the humidity containedin the air being highly prejudicial to the correct functioning of theCO—CO₂ catalyst.

Finally, the processing means 2 are constituted by an active carbonfilter 16, for removing any trace of taste and odour from the treatedair, and attached to the catalyst 14 by means of a pipe 17. The activecarbon filter 16 is also attached to an exit pipe 20 of the processingmeans 2.

The rehumidification means 4 of the treated air will now be described,still with reference to the sole FIGURE.

The rehumidification means 4 comprise an inlet pipe 18, attached to theoutlet pipe 20 of the treatment means 2 by a pipe 19.

At a point P situated on the inlet pipe 18, the latter is divided intotwo to form two parallel lines 22 and 24, which join at a point Q wherethey attach to an outlet pipe 26 of the rehumidification means 4.

Of the two lines 22 and 24, a dry line 22 is first evident, made up of aprincipal dry air pipe 28 on which is mounted close to the point Q ananti-return valve 30 with a known loss of load. This loss of load willpreferably be of the order of 300 mbar.

The other line situated between the points P and Q is a rehumidificationline 24. This line 24 comprises successively between points P and Q apipe for derivation of dry air 32, a water tank 34, as well as an airpipe saturated in humidity 36. Note that the pipe for derivation of dryair 32 communicates with a part of the tank 34 filled with water, whilethe air pipe saturated in humidity 36 communicates with a part of thetank 34 not including water. In other terms, a water level 37 inside thetank 34 is preferably maintained such that the water in the tank 34 isalways in contact with the pipe for derivation of dry air 32, but neverin contact with the air pipe saturated in humidity 36.

It is specified that a regulating valve 38 be mounted on the pipe forderivation of dry air 32, while an anti-return valve 40 is mounted onthe pipe of air saturated in n humidity 36, near point Q.

As mentioned hereinabove, the dry line 22 and the rehumidification line24 join up at point Q, to the main dry air pipes 28 and air saturated inhumidity 36 The pipes 28 and 36 are attached to the outlet pipe 26, onwhich is mounted a probe 42 for measuring the rate of humidity of thetreated air. The probe 42 is attached to pilot means 48, sensitive tothe signal emitted by the probe 42, and capable of piloting theregulating valve 38 mounted on the pipe for derivation of dry air 32.The installation 1 for production of respirable air functions asfollows.

The compressed air coming from the compression means enters theinstallation 1 via the pipe 9, as is indicated by the arrow A, thenfirst undergoes treatment by successively borrowing the followingelements: the pipe 9, the oil separator filter 6, the pipe 10, the dryer11, the pipe 12, the particle filter 13, the pipe 15, the catalyst 14,the pipe 17, the active carbon filter 16, and the pipe 20.

In this pipe 20 the air circulating inside is dry and treated, and isconveyed to the rehumidification means 4 by way of the pipe 19,connected to the inlet pipe 38.

When treated dry air arrives at point P, it is distributed both into themain dry air pipe 28, and also into the pipe for derivation of dry air32. The presence of distribution means, constituted in the embodimentdescribed by the regulating valve 38, fully controls the ratio betweenthe quantity of treated air passing through the main dry air pipe 28,and the quantity of treated air circulating in the pipe for derivationof dry air 32.

The air circulating in the main dry air pipe 28 does not undergo anyspecific treatment, and is conveyed only to the point 4 where it ismixed with the treated air originating from the rehumidification line24. On the other hand, the air circulating in the pipe for derivation ofdry air 32 transits via the water tank 34 where it is loaded withhumidity to saturation point, then rejoins point Q by way of the pipe ofair saturated in humidity 36. Note that the anti-return valve 40 isprovided so that the dry air coming from the main dry air pipe 28 doesnot enter inside the water tank 34.

In this way, the outlet pipe 26 contains a mixture of dry air and airsaturated in humidity, this mixture being adapted to obtain apredetermined rate of humidity of air produced by the installation 1. Infact, to obtain the desired proportions of dry air and air saturated inhumidity leading to the predetermined rate of humidity, the probe 42constantly controls, by means of pilot means 48, the opening of theregulating valve 38, and consequently authorises a limited and variablequantity of dry air coming from the inlet pipe 18 to pass through.Because of this, the more the desired rate of humidity is raised, themore the controlled opening of the regulating valve 38 is important.Note also that the probe 42 also controls the temperature of thesupplied air.

Permanent regulation of the valve 38 is also of interest when the rateof air from the installation 1 varies, this especially being the casewhen the number of operators breathing the air produced by theinstallation 1 increases or decreases. In such a situation, a change inthe rate of air inside the installation 1 can cause a change in thedistribution of treated dry air between the pipes 28 and 32, theconsequence of which could be to modify the rate of humidity in the airproduced circulating in the outlet pipe 26. However, since the probe 42constantly measures the rate of humidity at the outlet of theinstallation 1, it allows the opening of the regulating valve 38 to bereadjusted in real time, such that the resulting air can keep the samerate of humidity as that of the air produced when the number ofventilated suits connected to the installation 1 is different.

With such an installation 1, there is provision to obtain treated airwhereof the rate of humidity is constant, irrespective of the rate ofthe installation 1, this rate of humidity of the air being preferablybetween 40 and 50%.

The role of the anti-return valve 30 with known loss of load isessentially to create a difference in pressure between the main dry airpipe 28, and the pipe for air saturated in humidity 36. Such adifference in pressure tends to favour the passage of dry airoriginating from the main dry air pipe 28, in the outlet pipe 26. Byusing this particular arrangement, only the air coming from the pipe ofair saturated in humidity 36 rejoining the outlet pipe 26 is avoided,the effect of which is to excessively wet the probe 42, and to then makeit inoperative.

The treated and rehumidified air circulating inside the outlet pipe 26can thus exit the installation 1 (arrow B) at a controlled rate ofhumidity, and be redistributed to the ventilated suits of the operators.

According to a preferred embodiment of the invention, the installation 1comprises analysis means 44 for quantities of CO and CO₂ contained inthe air leaving the processing means 2. The analysis means 44communicate with the processing means 2 by way of a pipe 46, directlyattached to the outlet pipe 20 of the processing means 2.

The analysis means 44 verify permanently that the quantities of CO andCO₂ in the treated air do not exceed maximum values, preferablyconstituted by the values indicated in the European standard mentionedpreviously.

In the event where at least one of the maximum values is exceeded anddetected by the analysis means 44, the pilot means 48 are likely tocontrol one or more actions informing of the detected malfunctioning.

By way of example, the pilot means 48 can then control triggering asound and/or visual alarm which can be located at the intervention siteof the operators, or control a stop in the production of air from theinstallation 1, or a change in the source of compressed air, by tiltingfor example on an emergency compressor.

In addition, note that the pilot means 48 preferably comprise aninverter (not shown) producing at least one of the commands mentionedhereinabove, during a drop in the supply voltage from the installation1.

To secure the installation 1 even further, a reserve of treated air 50can be provided, preferably having a capacity of approximately 1000litres, fed by treated air by way of a pipe 52 communicating with thepipe 19 of the installation 1.

The reserve of air 50 communicates with the outlet pipe 26, preferablybetween the point Q and the probe 42, by means of a pipe 54 on which ismounted an electrovalve 56, kept closed during normal operation of theinstallation 1.

On the other hand, when the analysis means 44 detect a malfunction inthe installation 1, they are also able to control the closing of aneIectrovalve 58 mounted on the inlet pipe 18, and thus cut the influx ofair originating from the processing means 2. In addition, by controllingthe opening of the electrovalve 56, the pilot means 48 authorise thepassage of air stored in the reserve 50 through the pipe 54, in thedirection of the pipe 25 between the point Q and the probe 42.Deflecting to the reserve of treated air 50 allows the active operatorsto have available a sufficient quantity of air in their ventilatedsuits, so they can leave the work site in total security.

An additional alarm of the pneumatic alarm type 58 supplied by thereserve of air 50 can also be provided on the reserve of air 50, thisalarm 58 being particularly significant since it is capable offunctioning even during a break in power supply and a breakdown by theinverter.

The invention also relates to a process for producing respirable air foruse by an installation 1 such as that which has just been describedhereinabove.

The process comprises the successive stages of processing compressed airand rehumidification of the treated dry air. In the rehumidificationstage of the treated dry air, the distribution of treated dry air iscontrolled, between a dry line 22 and a rehumidification line 24, so asto obtain a mix of treated air at a predetermined rate of humidity.

It is understood that various modifications can be made by the expert tothe installation 1 for production of air and to the process, which havejust been described, solely by way of non-limiting examples.

1. A process for production of respirable air comprising the followingstages: treatment of compressed air comprising an operation for dryingthe air; rehumidification of the treated dry air, characterised in thatthe rehumidification stage of the treated dry air comprises an operationfor controlled distribution of the treated dry air on the one hand in arehumidification line (24), and on the other hand in a dry line (22). 2.The process as claimed in claim 1, characterised in that thedistribution of treated dry air is controlled by means of a regulatingvalve (38) mounted on the rehumidification line (24) and controlled bypilot means (48) sensitive to the signal emitted by a probe (42)measuring the rate of humidity, said probe (42) being mounted on anoutlet pipe (26), attached at one end to the rehumidification line (24),and at the other end to the dry line (22).
 3. The process as claimed inclaim 2, characterised in that a difference in pressure is createdbetween the rehumidification line (24) and the dry line (22), so as tofavour the passing of treated dry air coming from the dry line (22), inthe outlet pipe (26).
 4. The process as claimed in any one of thepreceding claims, characterised in that the processing stage ofcompressed air comprises the following operations: filtering ofcondensates in the compressed air; drying the air so as to eliminate anytrace of humidity in the air; filtering dust dislodged during the dryingoperation; transformation of carbon monoxide contained in the compressedair to carbon dioxide; filtering the air by means of an active carbonfilter (16).
 5. The process as claimed in any one of the precedingclaims, characterised in that the processing stage of the compressed airis followed by a permanent analysis stage of quantities of carbonmonoxide and carbon dioxide present in the treated air, then by an alertstage when the values of these quantities exceed maximum values to berespected.
 6. The process as claimed in any one of the preceding claims,characterised in that after the rehumidification stage of the treateddry air the air has a rate of humidity of between approximately 40 and50%.
 7. The process as claimed in any one of the preceding claims,characterised in that the rehumidified treated air is provided to supplyat least one ventilated suit of an operator carrying out dismantlingwork on nuclear installations.
 8. An installation (1) for production ofrespirable air comprising: processing means (2) for compressed aircomprising means for drying the air (11); rehumidification means (4) oftreated dry air; characterised in that the rehumidification means (4) ofthe treated dry air comprise a rehumidification line (24) and a dry line(22), as well as distribution means (38) for controlled distribution ofthe treated dry air in each of said lines (22, 24).
 9. An installation(1) as claimed in claim 8, characterised in that the rehumidificationmeans (4) of the treated dry air also comprise: an inlet pipe (18)containing treated dry air attached at one end to a main pipe of dry air(28) constituting the dry line (22), and at the other end to a pipe forderivation of dry air (32) belonging to the rehumidification line (24);an outlet pipe (26) attached ay one end to a pipe of air saturated inhumidity (36) belonging to the rehumidification line (24), and at theother end to said main dry air pipe (28) a water tank (34) belonging tothe rehumidification line (24), communicating at one end with said pipefor derivation of dry air (32), and at the other end with said pipe ofair saturated in humidity (36).
 10. The installation (1) as claimed inclaim 9, characterised in that the means for distributing the treateddry air are made up of a regulating valve (38) mounted on said pipe forderivation of dry air (32), said regulating valve being controlled bypilot means (48), sensitive to the signal emitted by a probe (42)measuring the rate of humidity, mounted on said outlet pipe (26). 11.The installation (1) as claimed in claim 9 or the claim 10,characterised in that the rehumidification means (4) of the treated dryair further comprise an anti-return valve (30) with known loss of load,mounted on said main dry air pipe (28).
 12. The installation (1) asclaimed in claim 11, characterised in that the anti-return valve (30)with known load loss causes a drop in pressure in the main dry air pipe(28) of around 300 mbar.
 13. The installation (1) as claimed in any oneof claims 9 to 12, characterised in that the rehumidification means (4)of the treated dry air further comprise an anti-return valve (40)mounted on said pipe of air saturated in humidity (36).
 14. Theinstallation (1) as claimed in any one of claims 8 to 13, characterisedin that the processing means (2) for compressed air comprise: an oilseparator filter (6) at 0.01 ppm; an adsorption dryer (11) with a dewpoint of −73° C.; a 1-micron particle filter (13); a catalyst (14) fortransforming carbon monoxide into carbon dioxide; an active carbonfilter (16).
 15. The installation (1) as claimed in any one of claims 8to 14, characterised in that it further comprises, at the outletprocessing means (2) for compressed air, analysis means (44) forpermanently controlling the quantities of carbon monoxide and carbondioxide present in the treated air.
 16. The installation (1) as claimedin claim 15, characterised in that the analysis means (44) communicatewith the pilot means (48) for controlling triggering a sound and/orvisual alarm, and/or deflection to a reserve of treated air (50), and/ora change in the source of compressed air.
 17. The installation (1) asclaimed in any one of claims 8 to 16, characterised in that it iscapable of supplying respirable air at a rate of humidity of betweenapproximately 40 and 50%.
 18. The installation as claimed in any one ofclaims 8 to 17, characterised in that it is attached to at least oneventilated suit of an operator carrying out dismantling work on nuclearinstallations.